The importance of the social sites is that people don't simply visit and then move on. They spend time on the sites - in fact, it would be true to say that some people are verging on addiction to these sites.
Social marketing offers tremendous opportunities - if you approach the sites in the right way (add value) and use the right tools (automate where possible).
The value of social media does not lie in backlinks since they are all "nofollow". You have to view the social media as:
a marketing channel in their own right - albeit a channel that funnels people to another channel, your website
part of your brand building approach
I have broken my overview down into the following sections:
business networks marketing
And to really make the most of social marketing opportunities, you should consider creating your own social site for your niche.